AT:

• variable name can be provided in the group by value structure as name and will be stored and returned by the backend
• the value is optional, if it isn't provided in the report update body it is not available
• structure change is documented in the wiki

Notes:
Right now, we only store the variable id in the groupby field of decision reports: https://github.com/camunda/camunda-optimize/wiki/Reports#input-variable

In order to correctly display the variable name instead of this id in report visualizations, it would be helpful to also store the variable name there. The name would be sent by the frontend when a report is saved, the backend only needs to store and return it